As a method for forming a thin film for electronic components or electromagnetic wave shields or patterning conductive circuits, there are generally known a method where a circuit is printed with a thermo-setting or thermoplastic conductive ink, followed by drying by heating and an etching method using a copper clad substrate.
The etching method is one kind of chemical processes and mainly applied to form a desired pattern shape on a metal surface. However, the etching method is generally problematic because processes thereof are complicated and a waste liquid has to be treated in a back-end step. Further, there is a problem that conductive circuits formed are weak to physical impact such as bending or the like, since the conductive circuits formed by the etching method are composed by only metal such as aluminum, copper or the like.
On the other hand, the conductive ink allows readily forming conductive circuits by printing or coating the conductive ink on a substrate, followed by drying and curing it. Therefore, a demand for the conductive ink is increasing recently from the viewpoint of foaming smaller and lighter electronic component, improving in the productivity or lowering the cost.
A thermosetting conductive ink includes a thermosetting resin and/or an inorganic material such as glass frit as a binder component. The thermosetting conductive ink, accordingly, has to be heated at a high temperature after coating or printing it on a substrate. In curing the thermosetting conductive ink by heating, much energy and time are needed, and a large floor space is also needed for setting units. This is not only uneconomical but also causes a large restriction as shown below.
That is, it is usually needed to heat the conductive ink that contains an inorganic material such as glass frit or the like as a binder component at 800° C. or more for forming a conductive circuit. Therefore, it can not be applied to a synthetic resin substrate. On the other hand, a conductive ink that contains a thermosetting resin as a binder component is applicable to a synthetic resin substrate. However as the conductive ink is needed to heat when cured, deformation of the substrate is caused by the heating in some cases. When a deformed printed wiring circuit is used, there is a problem that troubles are caused when a component is mounted in a following step.
The conductive circuit formed by using a thermoplastic conductive ink is much used in keyboards of personal computers and the like. However, as a substrate such as polyethylene terephthalate shrinks in a drying step of the conductive ink, a pre-treatment of the substrate such as annealing was necessary as countermeasures thereto. Further, it has defects that such a long time as 30 to 60 minutes is needed in the drying step and the conductive circuit obtained is not solvent-resistant.
On the other hand, an active energy ray-curable conductive ink, that is polymerized by irradiation of UV-rays, an electron beam or the like, does not contain or slightly contains a volatile organic solvent, and is cured instantaneously. As it is less in energy consumption, the study of the active energy ray-curable conductive ink is under active state from the viewpoint of reduction of energy consumption and environment preservation.
In recent years, as a method for forming a conductive circuit, there has been tried a method in which a dispersion containing a metal element or a metal element compound is applied on a base material such as a substrate to form a conductive or semi-conductive active energy ray-curable pattern or film and then irradiating it with an active energy ray. In Japanese Patent application laid open (JP-A) No. 2003-140330 there is described a method in which a radical generator is added to a dispersion to eliminate the dispersion and thereby develop the conductivity. Further, JP-A Nos. 2003-140330, 2001-64547, 2002-72468, 2003-110225, 2004-127529 and JP-W No. 2002-542315 describe examples of a conductive ink in which a compound to be cured by an active energy ray as a binder component of a conductive paste is used.
In JP-A No. 2003-110225, the conductive circuit having a volume resistance of an order of 10−6 Ω·cm is obtained. The conductivity, therefore, is high and further the usage range of the conductive circuit is wide. However, after radicals are generated from a radical generator by irradiation of UV-rays, heating at 250° C. for 30 minutes or more is necessary. From this, it is obvious that a metal therein is sintered by heat at the heating to develop the conductivity in the method described in JP-A No. 2003-110225. Accordingly, the heating at 250° C. or more is inevitable for developing excellent conductivity in this method. The heating step is unfavorable because it damages other electronic components and substrates.
All conductive inks described in JP-A Nos. 2003-140330, 2001-64547, 2002-72468, 2003-110225, 2004-127529 and JP-W No. 2002-542315 are conductive inks in which a composition containing an active energy ray-polymerizable compound is used as a binder. Printed conductive circuits formed by use of these conductive inks are instantaneously cured with an active energy ray such as an electron beam or UV-rays and therefore excellent in the productivity. However, any conductive circuits formed are high in resistance. Further, these documents do not describe a selection procedure of binder components for reducing the value of resistance. Accordingly, from these descriptions, it is insufficient to obtain suggestions for reducing the value of resistance, which is one of objects of the invention.
Furthermore, JP-A No. 2002-43739 discloses a conductive adhesive material in which a chlorinated polyester is used as a charge control agent. However the conductive adhesive material does not contain an active energy ray-polymerizable compound as a binder component. Therefore, when a thermoplastic resin is used as a binder substance, a heat treatment at a melting temperature or more thereof is necessary, and when a thermosetting resin is used, a heat treatment at a curing temperature or more thereof for drying and curing the conductive adhesive material. As the result, the productivity is poor and the economic efficiency is less as well.